1. Technical Field of the Invention
This invention relates to overhead projectors and, in particular, projection lenses used with such projectors.
2. Description of the Prior Art
Overhead projection lenses take many forms, the most conventional form in use today being a two-element projection lens assembly utilizing two spaced positive meniscus lenses, one to direct light toward a mirror reflector and the other to receive light from the mirror to project an image from a stage located below the projection lens assembly to a vertical projection screen or surface. The projected image is focused by movement of the projection lens assembly toward and away from the stage upon which the transparency to be projected is placed. Conventional two-element projection lenses are illustrated and described in such publications as U.S. Pat. Nos. 3,126,786 and 3,334,957.
Such conventional projection lenses as those described in the above patents encompass a focal length range of approximately 275 to 355 mm. For the magnification range through which this type of projection lens is used, a projection distance is obtained that places the overhead projector a few feet from the projection screen. For example, using a typical 350 mm focal length projection lens at 5.7.times. magnification, the projector is between seven and eight feet from the projection screen.
For many room and audience configurations, this distance between the projector and the screen results in partial blockage of the screen by the projector and the presenter. A better arrangement would result if the presenter and projector were moved to a position near the side and front of the audience, with the projection screen remaining approximately centered with respect to the audience. This would give the entire audience a clear view of the screen, yet enable the presenter to maintain eye contact with the audience. To achieve such an arrangement, the projection distance from the overhead projector to the screen must be increased to about one and one-half times the normal projection distance, without increasing the size of the projected image.
This longer projection distance requires a longer focal length projection lens than the conventional two-element assembly. For example, to provide a projection distance of between 11 and 12 feet at 5.67.times. magnification, a projection lens having a focal length near 525 mm would be required. For the type of projection lenses normally used in overhead projectors, this longer focal length results in an increase in the back focal distance of about 60%. The projection lens assembly is then incompatible with normal overhead projector focus adjustment ranges, and also with the focal length of the Fresnel condensing lens typically contained within the overhead projector. To overcome this problem, overhead projectors have been modified to include longer focusing post lengths, and longer focal length Fresnel lenses. These modified machines are usually awkward in appearance, difficult to transport and require larger diameter projection lenses.
U.S. Pat. No. 4,350,415 attempts to increase the projection distance of a conventional two-element projection lens assembly by adding a pair of auxiliary positive and negative meniscus lenses to increase its focal length from 355 mm to 530 mm. By introducing a telephoto ratio of 0.87, the back focus distance at 5.7.times. magnification is reduced to about 388 mm, at a projection distance of about 12 feet. However, the back focus distance is still too high to be compatible with conventional overhead projectors for smaller size screens. The lens assembly is, therefore, not usable with these overhead projectors over their entire operating range. Also, the resulting chromatic aberration caused by the auxiliary meniscus lenses using the same type of dispersive glass as the conventional projection lenses results in color fringing in the projected image.